In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using ordinary paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate (for example, refer to JP 2002-294105A, JP 2001-329199A, JP 8-157761A, JP 2003-183554A, JP 2004-75759A, JP 2003-138179A, JP 2003-147236A and JP 2004-115589A).
JP 2002-294105A discloses a recording solution to which an emulsion obtained by conducting polymerization in the presence of a reactive surfactant is added to prevent clogging of nozzles with pigments.
JP 2001-329199A discloses an ink composition containing a self-dispersible pigment and polymer particles to improve a high lighter-fastness and a rubbing resistance thereof.
JP 8-157761A discloses a water-based ink containing an oil film-forming component which is prepared by dissolving a water-insoluble resin therein to suppress color bleeding. JP 2003-183554A discloses a water-based ink which contains 10 to 1000 ppm of a phthalic diester and a latex in order to form images having a good gloss. JP 2004-75759A discloses a dispersion of colored fine particles in which a hydrophobic pigment, a hydrophobic polymer and a high-boiling organic solvent are micro-capsulated with an urethane group-containing resin.
JP 2003-138179A and JP 2003-147236A disclose pigment-containing inks for ink-jet printing which contain aliphatic acids and derivatives thereof. JP 2004-115589A discloses a water-based ink containing a self-dispersible pigment, a resin and an organic solvent exhibiting a solubility in water of 10% by weight or lower.
However, these conventional inks have failed to exhibit both a high optical density and an excellent high lighter-fastness to a sufficient extent.